The present invention relates to CRT display equipment, and in particular to a CRT display unit in which it is possible to significantly reduce electromagnetic radiation leaking from the front panel thereof in a simple and effective way.
In CRT display units, wherein image micro signals having a rectangular waveform and high frequency are received from a computer and are amplified to amplitudes large enough to be able to form an image on the front screen of the units, the input rectangular waveform image signals contain a large amount of harmonic components. Further, the higher the basic clock frequency of image signals becomes or the larger the amplified signal amplitude becomes, the greater the electromagnetic radiation becomes.
This electromagnetic radiation, when it penetrates into electronic devices disposed in the vicinity of the CRT display units, will cause electromagnetic interference and various malfunctions, so that it is necessary to adopt countermeasures to eliminate, as much as possible, any electromagnetic radiation generated in the CRT display units. As an example of such countermeasures, in Japanese Patent Laid-Open No. 58-7751 (1983), there is described an embodiment wherein a conductive film coated on the surface of the funnel portion of the CRT envelope is extended as far as the rim band around the panel section of the envelope to provide grounding. In another example of such countermeasures, as described in Japanese Patent Laid-open No. 62-267790 (1987), grounding is provided at two points, i.e. one point from a conductive film coating and the other point from the rim band, respectively.
As for the electromagnetic radiation leaking from CRT display equipment, various countries have set their own rules and regulations to control this problem. For example, in Japan and the U.S.A., control rules and regulations have been established against such EMI by the voluntary Control Council for Interference by Data Processing Equipment and Electronic Office Machines (VCCI), and the Federal Communications Commission (FCC).
These rules and regulations demand that the magnitude of such EMI radiation, when measured at a given distance from an EMI radiation source, not exceed limiting values set for particular frequency bands, and the standards imposed are stricter with the VCCI regulations than with the FCC regulations. In fact, the apparatus described in the above-mentioned Japanese Patent Laid-open No. 58-7751 (1983) has at least one frequency band where the VCCI rules and regulations cannot be conformed to, and the Japanese Patent Laid-Open No. 62-267790 (1987) narrowly measures up to the rules and regulations required with the smallest margin. Because EMI control is very sensitive and delicate, and even will accompany scattering in measured values for the same countermeasures taken, it is desirable for the regulation values to be cleared with plenty of margin.